There is a need for a compact see through data display capable of displaying image content ranging from symbols and alphanumeric arrays to high-resolution pixelated images. The display should be highly transparent and the displayed image content should be clearly visible when superimposed over a bright background scene. The display should provide full color with an enhanced color gamut for optimal data visibility and impact. A desirable feature is that the display should be as easy to wear, natural and non-distracting as possible with a form factor similar to that of ski goggles or, more desirably, sunglasses. The eye relief and pupil should be big enough to avoid image loss during head movement even for demanding military and sports activities. The image generator should be compact, solid state and have low power consumption.
The above goals are not achieved by current technology. Current wearable displays only manage to deliver see through, adequate pupils, eye relief and field of view and high brightness simultaneously at the expense of cumbersome form factors. In many cases weight is distributed in undesirable place for a wearable display in front of the eye. One common approach to providing see through relies on reflective or diffractive visors illuminated off axis. Microdisplays, which provide high-resolution image generators in tiny flat panels, often do not necessarily help with miniaturizing wearable displays because a general need for very high magnifications inevitably results in large diameter optics. Several ultra low form factor designs offering spectacle-like form factors are currently available but usually demand aggressive trade-offs against field of view (FOV), eye relief and exit pupil.
A long-term goal for research and development in HMDs is to create near-to-eye, color HMDs featuring:                a) high resolution digital imagery exceeding the angular resolution of standard NVGs over the entire field of view and focused at infinity;        b) a 80°×40° monocular field-of-view (FOV) HMD, or a 120°×40° binocular FOV HMD with 40° stereoscopic overlap at the center of the FOV;        c) a high see-through (≥90%) display with an unobstructed panoramic view of the outside world, a generous eye box, and adequate eye relief; and        d) a light-weight, low-profile design that integrates well with both step-in visors and standard sand, wind and dust goggles.        
Although the imagery will be displayed over a certain field of view, the panoramic see-through capability may be much greater than this and generally better than the host visor or goggles. This is an improvement over existing NVGs, where the surrounding environment is occluded outside the 40° field of view.
One desirable head-worn display is one that: (1) preserves situational awareness by offering a panoramic see-through with high transparency; and (2) provides high-resolution, wide-field-of-view imagery. Such a system should also be unobtrusive; that is, compact, light-weight, and comfortable, where comfort comes from having a generous exit pupil and eye motion box/exit pupil (>15 mm), adequate eye relief (≥25 mm), ergonomic center of mass, focus at infinity, and compatibility with protective head gear. Current and future conventional refractive optics cannot satisfy this suite of requirements. Other important discriminators include: full color capability, field of view, pixel resolution, see-through, luminance, dynamic grayscale and low power consumption. Even after years of highly competitive development, HWDs based on refractive optics exhibit limited field of view and are not compact, light-weight, or comfortable.
Head-mounted displays based on waveguide technology substrate guided displays have demonstrated the capability of meeting many of these basic requirements. Of particular relevance is a patent (U.S. Pat. No. 5,856,842) awarded to Kaiser Optical Systems Inc. (KOSI), a Rockwell Collins subsidiary, in 1999, which teaches how light can be coupled into a waveguide by employing a diffractive element at the input and coupled out of the same waveguide by employing a second diffractive element at the output. According to U.S. Pat. No. 5,856,842, the light incident on the waveguide needs to be collimated in order to maintain its image content as it propagates along the waveguide. That is, the light should be collimated before it enters the waveguide. This can be accomplished by many suitable techniques. With this design approach, light leaving the waveguide may be naturally collimated, which is the condition needed to make the imagery appear focused at infinity. Light propagates along a waveguide only over a limited range of internal angles. Light propagating parallel to the surface will (by definition) travel along the waveguide without bouncing. Light not propagating parallel to the surface will travel along the waveguide bouncing back and forth between the surfaces, provided the angle of incidence with respect to the surface normal is greater than some critical angle. For BK-7 glass, this critical angle is ˜42°. This can be lowered slightly by using a reflective coating (but this may diminish the see through performance of the substrate) or by using a higher-index material. Regardless, the range of internal angles over which light will propagate along the waveguide does not vary significantly. Thus, for glass, the maximum range of internal angles is ≤50°. This translates into a range of angles exiting the waveguide (i.e.; angles in air) of <40°; generally less, when other design factors are taken into account.
To date, SGO technology has not gained wide-spread acceptance. This may be due to the fact that waveguide optics can be used to expand the exit pupil but they cannot be used to expand the field of view or improve the digital resolution. That is, the underlying physics, which constraints the range of internal angles that can undergo total internal reflection (TIR) within the waveguide, may limit the achievable field of view with waveguide optics to at most 40° and the achievable digital resolution to that of the associated image.